Seeing the Unseeable

Black holes really are black. Their gravity prevents any matter or energy from escaping, so there is no way to see them directly. Yet astronomers have discovered plenty of black holes through their influence on the matter, energy, and even space around them.

Astronomers discover black holes by looking for their effects on the stars, gas, and space around them.

Because a black hole is both massive and compact, it exerts a strong gravitational pull on the material around it. Astronomers can deduce the presence of a supermassive black hole in the core of a galaxy by measuring the velocities of stars that orbit the black hole. A more-massive black hole will accelerate nearby stars to greater speeds, so the velocities of stars can reveal not only the presence of a black hole, but its mass as well.

They can also deduce the presence of a supermassive black hole by measuring the velocities of clouds of gas that orbit the black hole. The gas emits radio waves and other forms of energy. By measuring changes in the speed of the gas clouds toward or away from Earth, astronomers measure their speeds. Because gas usually follows circular orbits, it is easier to use them to measure the mass of the black hole they orbit.

Astronomers use these same basic techniques to discover stellar-mass black holes (those that are a few times as massive as the Sun).

Many star-mass black holes are members of binary systems, which means they have companion stars. In a binary that contains a black hole and another type of star (one that produces visible light or other forms of energy), the orbital speeds of the two component stars is much greater than in a system with two "normal" stars (stars that are similar to the Sun). Measuring the orbital speeds of the two components in a binary system, along with the distance between the stars, reveals the system's total mass. Using other techniques, astronomers can determine the mass of the luminous companion. By subtracting that from the system's total mass, they can determine the mass of the dark companion, which reveals whether it is a black hole or a less-dense object like a neutron star. This technique is like the one that astronomers use to deduce the masses of planets in solar systems other than our own.

In addition, many black holes are encircled by disks of superhot gas, called accretion disks. In the case of a star-sized black hole, the gas usually comes from a nearby companion star; the black hole's powerful gravity pulls gas off the surface of the star. In the case of a supermassive black hole, the source is large clouds of gas in the crowded core of a galaxy, or stars that pass close to the black hole and are torn apart by its gravitational pull. As the gas spirals into the black hole, it forms a wide, flat "accretion disk." The gas moves faster and faster as it spirals closer, so it's heated to millions of degrees. At such temperatures, the gas radiates most strongly in ultraviolet or X-ray wavelengths. These wavelengths are blocked by Earth's atmosphere, so only telescopes in space can detect them.

In many cases, these telescopes can measure the speed of the gas at different distances from the black hole, which provides a good measurement of the mass of the central object, which provides a high level of confidence that the object is a black hole.

In other cases, though, the object is so small and distant that telescopes cannot see details in the accretion disk. In these cases, astronomers deduce the source of the X-ray or ultraviolet energy from the overall characteristics of the energy. This method has yielded detections of thousands of possible black holes in many different galaxies.

But this technique doesn't provide enough details to give an accurate measure of the black hole's mass and, in some cases, whether the systems actually contain black holes; different types of objects could produce the X-ray or ultraviolet energy. Furthermore, since astronomers only measure the total X-ray energy, they have to rely on theoretical models of how those x-rays are produced to determine a black hole mass. Thus, it is not a direct measure of the mass. Confirmation of these possible black holes may await future orbiting telescopes that can see the universe with greater clarity than current instruments.

Finally, if a star or galaxy passes directly behind a black hole as seen from Earth, the black hole's gravity will distort and amplify the light of the background object. Astronomers are conducting several searches for black holes using this technique. The same technique can lead to detections of planets in distant star systems.